The mineralized tissues, dentin and bone, are comprised of an organic matrix in which the mineral phase becomes embedded. There appears to be a relationship between structural and compositional features of the matrix and the pattern and extent of normal deposition of the mineral phase within the matrix. Further, there appears to be specific interactions between the matrix components themselves which differentiate the matrix from that of the non- mineralizing soft tissues. The objectives of this research program are to determine the nature of those features of the matrix which differentiate the hard and soft tissues, and to understand how the matrix relates to and regulates the deposition, struture and retention of the mineral phase. These broad objectives devolve into four specific problems: (1) the identification and characterization of the matrix proteins (collagen, acidic structural glycoprotein, phosphoglycoprotein); (2) interactions between the matrix components (specifically in dentin, collagen-phosphoprotein covalent bonding); (3) the physical properties of the matrix proteins and their analogs with special regard to Ca ions and phosphate ion interactions; and (4) the biosynthesis of the non-collagen matrix components and their localization at the mineralizing front. Comparisons between bone and dentin demonstrate the similar roles of acidic- and phosphoglycoproteins in these tissues. Special attention i being given to the presence of a covalently linked collagen-phosphoprotein conjugate and to the dentin phosphoprotein. Bibliographic references: Localisation of plasma alpha glycoprotein in mineralising human bone. I.R. Dicson, A.R. Poole and A. Veis. Nature 256, 430 (1975). Evidence for abnormality of Bone-Matrix Proteins in Osteogenesis Imperfecta. I.R. Dickson, E.A. Millar and A Veis. Lancet, No. 7935, 1975, Vol. 2, 586-587.